Devices with a slate form factor such as tables, smartphones, media players and e-book readers are hard to hold for long periods of time. External stands can alleviate this problem in many situations.
Touch-input and pen-input methods are limited ergonomically when used for extended periods of time. External physical keyboards alleviate this problem for text entry. Easels or stands that hold the tablet can prevent the fatigue of constantly holding the tablet with one hand while pushing against it with the other hand for input directly with fingers or with a stylus.
Physically attaching accessories to mobile electronic devices, such as carrying cases with integrated stands, cases with integrated keyboards and/or cases with integrated pen-holders add weight, thickness and complexity to such devices and undermine their mobility virtues. Most of these accessories provide extra functionality but some have minimum stability when using them in places other than on a table-top. They also don't have very good grip to be placed on an inclined and/or uneven surface such as on a lap while sitting.
Most accessories require extra hardware components such as hinges and multiple moving parts to achieve multiple screen angles of the tablet. These accessories also add substantial complexity, size, weight, and possible failure points.